The utilization of heterosis of crops must be premised on obtaining F1 hybrid seeds at low cost, so that a user can buy hybrid seeds and plant hybrid rice to obtain yield and benefits. Said hybrid seeds are produced by two parents having hereditary stability and different genotypes through sexual hybridization (hybrid seeds F1 has genotype heterozygosis and completely identical expressivity). Plant heterosis has universality, and whether the heterosis can be effectively utilized largely depends on the cost of hybrid seed production and the purchasing capacity of users. Hybrid rice has been widely popularized successfully for over 30 years in China, and in order to obtain higher yield potential of hybrid rice, improving the grain weight of rice is commonly used in present, which increases the seed quantity (weight) and therefore increases the seed buying cost.
Since heterosis utilization of hybrid rice is successfully realized in China in 1974, the method of interval sowing of male parent (restorer line) and female parent (CMS or GMS line) and respectively obtaining the male parent and the female parent has been used all long, which not only has complex operation technique but also increases the labor cost, and during seed production, the male parent which is higher than the female parent after gibberellin (GA3) treatment for better cross pollination, the higher male parent is liable to lodging, and lodged plants or a minority of tiller ears are difficult to clear away in firstly harvest of male parent resulting in reduction of the purity of hybrids (mixed with a part of restorer line).
In the present hybrid seeds F1 production in China is according to period from seeding to heading between CMS or GMS line (female parent) and restorer line (male parent), interval sowing and interval or simultaneous transplanting are carried out, the male parent is immediately cut off after the completion of flowering, or the male parent is firstly cut off about 25 days after flowering (even the male parent is immediately cut off after the completion of flowering for preventing hybrid mixing with the male parent), and finally hybrid seeds of the female parent pollinated from restorer line are harvested after being grown. This hybrid rice production has been used for 35 years since 1974, and laborers for the seed production are more than common rice production by 3-4 laborers per mu (1 mu=0.067 Hectare) in merely interval sowing, interval or respectively transplanting of male and female parents and interval or respective harvest of the male and female parents.
In order to improve hybrid rice seed production efficiency, scientific researchers have done a lot of research, for example, the patent of China (CN200710070995) discloses a method for improving the efficiency of seed multiplication and production of hybrid rice by utilizing double-grain mutant, which improves the seed production efficiency by using double-grain mutant to improve CMS line; the patent CN99101907 discloses a hybrid rice seed production method, which improves the purity of seed production through chemical emasculation and weeding gene introduction. However, these methods are essentially different from the application.
A small grain mutant (ZH-sg, 1000-grain weight is about 20 g) obtained from r-ray mutagensis is under control of single recessive gene sg which has no bad effects on rice plant height, tillering capacity, photosynthetic capacity, spikelet fertility, grain shape, etc. The characteristic of small grain is controlled by single recessive gene sg, and hybrid seeds F1 produced by CMS or GMS line improved from the mutant are small grains. The small grain characteristic is unsuitable for improving yield potential of conventional rice seed production but completely feasible for the improvement of hybrid rice CMS or GMS line. Currently, there is not any report on utilizing small grain recessive mutation to improve CMS or GMS line, neither report nor invention on utilizing the difference of grain size to carry out mixed harvest and separating hybrid seeds from male parent according to the size of seeds by means of a mesh screen.